Method of reducing center porosity in ingot casting

ABSTRACT

A method of reducing porosity in the center of ingots promoting a uniform temperature gradient, and decreasing alloy segregation by vacuum stream degassing into an ingot mold and simultaneously gas purging from the center of the ingot stool to facilitate a thermal circulation.

United States Patent Inventor Charles W. Fink] 2011 N. Southport, Evanston, Ill. 60614 Appl. No. 73l,72l Filed May 24, I968 Patented June 1, I97] METHOD OF REDUCING CENTER POROSITY IN INGOT CASTING H 13,5s1,s03

[56] References Cited UNITED STATES PATENTS 3,084,038 4/l963 Finkl 266/34VDX 3,236,635 2/l966 Finkl 266/6 l X 3,208,l l7 9/ l 965 Goedecke et a]. 164/56 2,837,790 6/1958 Rozian 164/64 FOREIGN PATENTS l,237,404 6/l 960 France 266/34VD 796,369 6/1958 Great Britain 266/34VD Primary Examiner-J Spencer Overholser Assistant ExaminerVemon K. Rising Attorney-Parker, Carter & Markey ABSTRACT: A method of reducing porosity in the center of ingots promoting a uniform temperature gradient, and decreasing alloy segregation by vacuum stream degassing into an ingot mold and simultaneously gas purging from the center of the ingot stool to facilitate a thermal circulation.

METHOD OF REDUCING CENTER POROSITY IN INGOT CASTING This invention relates generally to the casting of steel, and particularly to a method of establishing a more uniform temperature gradient and ensuring less alloy segregation within an ingot, such as a forging ingot, by simultaneous vacuum degassing and casting. It is well known, for example, that the alloy content of an ingot may vary appreciably from top to bottomcarbon variations of five points in large forging ingots are well known for example-and this invention is directed to overcoming this problem.

Essentially the invention consists of creating a thermal circulation within an ingot mold or other casting forming receptacle by a number of manipulative steps, some of which may be optionally performed, but all of which are additive in ef feet. The final product will be a degassed steel ingot in which the gaseous voids in the region of porosity in the ingot, if they exist, will be of a size which will be readily healable by subsequent forging operations.

Accordingly, a primary object of the invention is to provide a method of castingin which a thermal circulation is created within the casting receptacle which has a generally downward component of movement adjacent the periphery thereof and a generally upward component of movement in the center area thereof, which circulation is maintained for a period of time sufficient to flush out substantially all of the undesirable gases which heretofore have contributed to the forming of gaseous voids in the center or pipe area of the ingot. Such a thermal circulation will likewise tend to flush to the surface of the receptacle the refractory particles, oxides, and other foreign matter which, as is well known, tend to be squeezed toward the center of the receptacle due to the inherent freezing pattern which is in a radially inwardly direction.

Another objectis to create a circulation as above described by stream degassing molten metal from a suitable source into an ingot mold and simultaneously bubbling a minimal quantity of gaseous agent upwardly through the mold which results in a minimal temperature drop attributable to gaseous agent.

Another object is to provide a method of casting as above described in which the flow of gaseous purging agent may be initiated immediately upon commencement of deposition of metal to the ingot mold.

Yet another object is to provide a method of casting as above described consisting of a number of separate, yet interrelated procedures no one of which acts to inhibit the establishment of circulatory movement by the gaseous purging agent.

Yet another object is to provide a method as above described which is nondestructive of the mold and the gaseous purging system, and permits, under suitable conditions, reuse of the refractory portions of the gaseous purging systems.

Another object is to provide a method. in which charge material additions, including alloying materials, may be made at any desired time and thoroughly mixed in the ingot without materially increasing the total treatment time.

And a further object is to provide a method in which close temperature control of the ingot may be maintained and provision made for adding heat to the ingot during the pouring process if for any reason this becomes a requirement.

Although the. invention may be carried out with a wide variety of apparatus, it will be helpful to visualize the method of the present invention in terms of a conventional ingot mold of the type widely used today in the melt shops of electric furnace steel producers. Such an ingot mold has a metal wall of substantial thickness, which wall serves as a heat sink as will appear hereinafter. It will also be helpful to visualize a conventional source of molten metal, such as a bottom pour ladle,

.and a vacuumtight pouring hood or collar between the upper open end of the ingot mold and the bottom area of the metal ladle about the discharge spout. Obviously any suitable vacuum system, such as a steam ejector system or a mechanical pump system, may be employed to create and maintain a Alternately, an argon filled vacuumtight vessel or enclosure may be employed, and the system operated at a subatmosphere pressure. Any gas which performs the same function as argon in this environment may, of course, be employed. If desired, and depending on the equipment available, the ingot mold or other container into which the molten metal is to be deposited may be placed entirely inside an airtight vessel in which event the mold or container would not form a part of the controlled atmosphere enclosure.

The only ingot mold modification required is the addition of means for admitting gaseous agents to the interior of the ingot mold through the central area of the bottom thereof. At this stage of development of the art such devices have become well known of which the construction shown in FIG. 4 of US. Pat. No. 3,236,635, is a typical example. Such a device may be placed in the center of the ingot stool.

Prior to the commencement of operations the vacuum hood or collar is placed on the top of the ingot mold and the supply ladle with a quantity of molten metal therein is positioned above the vacuum collar and a vacuumtight seal formed between the mating surfaces. The porous purging plug located in the bottom of the ingot mold will, of course, be valved off so that a vacuumenvironment can be created within the ingot mold prior to the commencement of pouring.

After assemblage of the components mentioned above and the creation of a vacuum within the ingot mold and pouring collar, the stopper rod or other flow control device of the supply ladle is opened to permit metal from the supply ladle to be admitted into the mold. By maintaining a relatively low vacuum in the ingot mold, for example on the order of several millimeters of mercury or less, and even down to about 1 or 2 millimeters of mercury, the pouring system will be dispersed radially outwardly as it leaves the discharge spout of the supply ladle, and the molten metal will pass in the form of molten droplets into the ingot mold. If desired, the streaming collar of the design illustrated in FIG. 3 of US. Pat. No. 3,084,038 may be employed to facilitate and direct the streaming metal.

Preferably the degree of vacuum and the relative spacing between the supply ladle discharge spout and the interior of the ingot mold are so correlated one to the other that at least a substantial portion of the downwardly falling stream will be dispersed radially outwardly. Preferably the spray of droplets will be relatively uniformly dispersed so that the small area on the mold bottom beneath the discharge spout which is in axial alignment with the axis of the discharge spout does not receive the full force and effect of the downwardly falling stream of molten metal. When teeming under atmospheric conditions for example, the pouring stream of metal is relatively tight with the result that a severe erosion problem is created in the area of the stool upon which the tight pouring stream impinges. The above described procedure, by way of contrast, eliminates or at least substantially reduces this effect which would be especially undesirable in the present system wherein a relatively erodable, porous refractory material may be disposed generally beneath the discharge spout of the supply ladle.

Simultaneously with the streaming of the molten metal into the ingot mold a gaseous agent is admitted to the mold through the porous member in the central area of the bottom thereof. Argon, helium, or dry air are preferred materials. Even reactive gases such as CCL, may be employed provided, .of course, the necessary safety precautions are taken. If desired, however, any of the inert gases or carbon monoxide may be employed, the only requirement being that the gaseous agent beinert with respect to the molten metal undergoing treatment insofar as the functioning of the gaseous agent as a circulation aid is concerned.

Since the pressure of the gas at the point of emission must be greater than the ferrostatic head of the metal above that point, and the pressure in the tank may be assumed to be approximately 5 millimeters mercury absolute, it can at once be appreciated that a tremendous expansion of the gas will take place. If the head of the metal is on the order of about 1 atmosphere for example each cubic unit of the gaseous agent will expand over 1,400 times its original, entering volume. Because of this great expansion only a minimum amount of gaseous agent will be required as contrasted with the amount required if the surface of the metal were at atmospheric pressure. The above-described expansion will result from the pressure differential alone, and there may be even a further expansion due to the rise in temperature of the purging agent.

The simultaneous admission of molten metal to the ingot mold at locations on or adjacent the internal wall thereof and the upward passage of the gaseous agent will induce a thermal circulation within the ingot mold which has a generally downward component of movement adjacent the interior wall of the ingot mold initially, and thereafter adjacent the radially inwardly solidifying skin, and a generally upward component of movement along the path of travel of the gas bubbles. That is, the molten metal in contact with the interior wall of the ingot mold will chill much more rapidly than the molten metal in the center thereof because of the heat absorption capacity of the ingot mold walls. The chilled metal adjacent the walls will therefore tend to move downwardly. The upwardly traveling purging gas bubbles will tend to create a movement of the metal in the central region upwardly and consequently the thermal circulation will be established. Because of the large expansion ratio of the purging agent only a minimal amount of said agent is required and the cooling effect of the agent on the molten metal will be relatively insignificant. ln fact, the cooling effect on the purging agent on the metal in the central region will be considerably less than the cooling effect on the ingot mold on the metal in contact with it. As a result there will be a temperature differential across the area of the surface of the metal in the ingot, the temperature being hottest in the central region and coolest at the regions adjacent the internal periphery of the mold.

Experience will dictate how long the upward passage of the gaseous agent should be maintained. Since the molten metal is resting in the receptacle in which it will freeze it is not necessary to stop the process at a point which makes provision for any further processing. Accordingly, the gaseous purging agent may be maintained for so long as it helps induce thermal circulation. It may for example be preferable to terminate purging at the conclusion of teeming. When the gaseous agent is shut off, the upward inertia of the molten metal will be sufficient to carry the gas in the metal at that time upwardly to the surface and discharge through the vacuum system. It will be understood that the gaseous agent will also function to flush refractory inclusions to the surface area where they can be removed with the cropped portion of the hot top.

Several additional steps may be employed in conjunction with the above described process, all of said additional steps being additive in nature.

If desired, heat can be added to the metal in the ingot mold by burning carbon monoxide to carbon dioxide before entering the mold. The carbon dioxide and other gaseous products of combustion may be admitted through the porous gas admission element. The addition of the gaseous products of combustion will have two effects. Firstly, the hot gas will further increase the mechanical agitation and circulation provided by the above-described gaseous agent and secondly, the additional heat will at least slow down and possibly increase the temperature of the molten metal coming in contact with it. Both these effects increase the thermal circulation resulting from the center portion of the mass being at a substantially higher temperature than the peripheral portions thereof.

Further, if desired, alloys may be added at any desired time in the cycle. The charged materials, which may include alloying materials, may be added to the downwardly falling stream by any conventional means, such as an alloy addition device of the type illustrated in US. Pat. No. 3,145,095.

Alternately, the charge material, including alloy addition materials if desired, can be pulverized and entrained with the gaseous agent prior to its passage into the metal through the porous purging agent.

lfdesired, reactive gases can be used for further deoxidation or dehydrogenation. Thus, for example, CCL, can be added. This gas reacts with the oxygen and hydrogen within the molten metal to produce carbon monoxide and hydrochloric acid.

Any combination or all of the latter described steps may be practiced in conjunction with the earlier described steps, the exact choice of procedure being dependent upon the product desired and the operating conditions.

The resultant product will be an ingot having good homogeneity and little or no gaseous voids in the pipe region. Those voids which may be present will be of a size which can be readily healed by subsequent forging. lf alloy materials have been added as above described the ingot will have a relatively uniformly distributed alloy content and segregation will, therefore, not be a major problem.

It will also be noted that the admission of the molten metal into the ingot in finely divided droplet form eliminates the possibility of undesirable wash or erosion of the refractory components of the system and consequently it may be possible to cast several before replacement of the purging element is required.

Although a preferred embodiment and various modifications have been described, it will be understood that the foregoing description is intended as exemplary only, for various modifications will at once be apparent to those skilled in the art.

Accordingly, it is intended that the scope of the invention be limited not by the scope of the foregoing description but solely by the scope of the hereinafter appended claims.

lclaim:

l. A method of reducing porosity in the pipe area of an ingot, said method including the steps of forming a zone of subatmospheric pressure between a source of molten steel and an ingot mold having an ingot stool,

said subatmospheric pressure being sufficiently low to cause dispersal of a downwardly passing stream of molten steel from the source into a spraylike form, teeming molten steel from said source into the ingot mold in a dispersed, spraylike form,

passing a purging agent upwardly through the molten steel in the ingot from an emission area located in the central portion of the ingot stool,

said purging agent being admitted at a rate sufficient to establish a thermal circulation within the ingot mold having an upward component of movement in the central portion which is at least partially induced by the upward passage of the purging agent, and a downward component of movement in the peripheral area of the ingot mold which is at least partially induced by the chilling effect of the ingot mold,

maintaining the upward passage of the purging agent, and

hence the thermal circulation, for a period of time sufficient to substantially decrease alloy segregation and promote a uniform temperature gradient by modifying the conventional radially inwardly directed freezing pattern,

terminating said purging agent treatment, and

freezing said molten steel in said ingot mold.

2. The method of claim 1 further characterized in that the heat is added to the steel by blowing hot products of combustion upwardly through the molten steel from the center portion thereof.

3. The method of claim 1 further including the step of making charge material additions to the molten steel in the mold by entraining said charge materials in finely divided form in the gaseous agent passing upwardly through the molten steel.

4. The method of claim 3 further characterized in that the charge materials include alloying materials, and further including the step of continuing the gaseous agitation phase of the thermal circulation until the charge materials are distributed throughout the molten steel to thereby compensate for establishing an initial temperature differential over a cross alloy segregation. section of the molten steel by 5. The method of claim 1 further including the step of directing a portion of the entering steel against the sidewalls making charge material additions to the molten steel in the of [he ingot mold Whareby the p iph f area of the P mold by adding Said charge materials to the downwardly of molten steel in the ingot mold lS Initially chilled more falling Stream. rapidly than the molten steel in the central area.

6. The method of claim 1 further including the step of 

2. The method of claim 1 further characterized in that the heat is added to the steel by blowing hot products of combustion upwardly through the molten steel from the center portion thereof.
 3. The method of claim 1 further including the step of making charge material additions to the molten steel in the mold by entraining said charge materials in finely divided form in the gaseous agent passing upwardly through the molten steel.
 4. The method of claim 3 further characterized in that the charge materials include alloying materials, and further including the step of continuing the gaseous agitation phase of the thermal circulation until the charge materials are distributed throughout the molten steel to thereby compensate for alloy segregation.
 5. The method of claim 1 further including the step of making charge material additions to the molten steel in the mold by adding said charge materials to the downwardly falling stream.
 6. The method of claim 1 further including the step of establishing an initial temperature differential over a cross section of the molten steel by directing a portion of the entering steel against the sidewalls of the ingot mold whereby the peripheral area of the pool of molten steel in the ingot mold is initially chilled more rapidly than the molten steel in the central area. 